While it has been shown that coating compositions such as those disclosed in the U.S. Pat. No. 4,548,644 to Mosser et al and assigned to the assigne of the present invention are suitable for dip-spin application to small parts such as fasteners, the thixotropic nature of the coatings causes problems when applied over fasteners which contain blind holes, fine threads and recessed heads, like Phillips crossheads, Torx.RTM., and other types of fasteners. These problems arise from the fact that due to the low shear forces exerted in the aforementioned areas, the thixotropic nature of the compositions does not allow full return of the coating to its liquid state thus providing the good flow and leveling exhibited on adjacent unencumbered areas of the fastener being coated. This lack of flow and leveling in these areas often gives rise to excessive build-up of coating, usually resulting in coated parts which are nonuseable, that is, rejected parts.
While there are physical means employed to facilitate the proper flow and leveling, such as fewer parts processed at a given time, increased spin time and centrifugal force and use of inserts to reorient parts, these means have only shown marginal improvement in solving the aforementioned problem. When employed in production operations, these means can diminish the throughput capacity for bulk coated parts, thereby defeating the purpose for which bulk processing is intended. Adjustments in the compositions within the spirit of the invention of the Mosser et al patent to allow lower viscosities also shows some improvement, but not yet such improvement as is required for various industrial applications.
During research and development, applicant has attempted to overcome the lack of flow and leveling in the compositions by incorporating wetting agents, i.e. surface tension modifiers, surfactants, to improve the flow and leveling. These wetting agents can be nonionic, anionic or cationic. The Mosser et al patent discloses the use of nonionic surfactants to establish what is referred to as the thixotropic network. The acid stable nonionic surfactant is capable and does form a three dimensional, crosslinked network with an amorphous silica. The nonionic surfactant is present in the composition at relatively low concentrations. Larger amounts of the nonionic surfactant adversely affect the consistency of the desired composition. Because the compositions are water based and therefore highly polar, it was demonstrated that nonionic species could be used for the practice of making compositions of the Mosser et al patent and that anionic and cationic wetting agents were not useful in establishing the thixotropic network.
Through experimentation, applicant has found that the addition of an anionic surfactant to the composition of the Mosser et al patent, in an attempt to reduce the surface tension of the composition, resulted in the viscosity previously established by the practice of the Mosser et al patent to be substantially lowered so as to render the composition substantially unusable for practical purposes. In these experiments, sodium 2-ethyl hexyl sulfate was used.
Another anionic material based on polysulfonic acid designated by the manufacturer (Henkel Corp., Minneapolis, Minn.) as a thickener and fluid friction reducer was added by applicant to the composition of the Mosser et al patent. This addition resulted in a significant increase in viscosity. When used alone in a precursor of the composition disclosed in the Mosser et al patent without the nonionic surfactant and silica, the polysulfonic acid thickened the material. To obtain viscosities that were suitable for good coating film development, high levels of the polysulfonic were required, giving the coated articles brittle films and marginal adhesion.
The present invention provides a coating composition which exhibits improved viscosity stability and improved wettability of intricate surfaces such as fine threads and recessed heads particularly when applied by dip-spin methods to small metal parts such as fasteners. The invention results in better flow and leveling of the compositions and less rejected unusable coated parts.